The invention relates to a pneumatic brake booster, in particular for motor vehicles, comprising a control valve for controlling a pneumatic pressure difference, a force input element for actuating the control valve, a force output element for transmitting the braking force, a reaction disk of elastomeric material disposed between the force output element and the force input element, and a unit disposed between the reaction disk and the force input element and acting upon the reaction disk for altering the force transmission ratio between the force input element and the force output element in dependence upon the force exerted upon the force input element, wherein the unit comprises an inner piston and an outer piston surrounding the radially outer side of the inner piston and displaceable relative to the inner piston.
Such a brake booster designed as a vacuum brake booster for a motor vehicle is known from EP 0 705 190 B1, and corresponding U.S. Pat. No. 5,699,713 which is incorporated by reference herein.
Normally, pneumatic brake boosters have a fixed force transmission ratio, i.e. an actuating force introduced by means of the input element into the brake booster is transmitted from a smaller area associated with the input element to a larger area associated with the force output element. This occurs mostly via a so-called reaction disk of elastomeric material, which behaves in a manner similar to a liquid. Under certain circumstances, e.g. in an emergency braking situation, it is however desirable to provide a user with as much braking force assistance as possible so that from a specific input force as high a brake pressure as possible can be generated.
In the cited EP 0 705 190 B1 it is therefore proposed that, after a specific input force has been exceeded, the area acting upon the reaction disk be reduced by means of the unit for altering the force transmission ratio. Said reduced area penetrates further into the elastic reaction disk compared to the non-reduced area (given the same input or actuating force) so that the control valve of the brake booster opens to a correspondingly greater extent, thereby resulting in a correspondingly higher differential pressure in the brake booster and hence in an increased force amplification.
In the brake booster known from EP 0 705 190 B1, the inner piston and the outer piston of the unit for altering the force transmission ratio act directly upon the reaction disk. As a result of the two pistons acting directly upon the reaction disk a fine dust of abraded elastomeric material arises. A gap, which is disposed between a stop for the outer piston and the surface of the outer piston facing said stop and is required for reliable operation of the unit for altering the force transmission ratio, is progressively used up by the dust collecting inside the device for altering the force transmission ratio. Once the gap is totally used up, it is no longer possible after a high-pressure braking operation for the brake pressure to be cancelled in a controlled manner. Rather, braking force assistance generated by the brake booster terminates only when the input force summoned up by the driver is sharply reduced. The reason for this is that, when the gap is used up, the hydraulic reaction forces acting upon the outer piston are introduced not into the force input element but entirely into the control valve housing.
The fact that despite a reduction of the input force summoned up by the driver the braking effect hardly alters is perceived by the driver as a deterioration of the actuating performance of the vehicle brakes. The driver gets the impression that his influence upon the performance of the vehicle brakes is decreasing, which may lead to critical driving situations.
The object of the invention is to provide a pneumatic brake booster of the type described initially, the actuating performance of which is less strongly dependent on wear phenomena.
Proceeding from a brake booster of the described type, said object is achieved according to the invention in that on the one hand the inner piston has, radially at the outside, a depression for receiving material particles which is covered by the outer piston and in that on the other hand the outer piston has, radially at the inside, a depression for receiving material particles which is covered by the inner piston. Material particles, which migrate counter to actuating direction and are caused e.g. by wear of the reaction disk, are consequently collected in the depression of the inner piston before they are able to deposit inside the unit for altering the force transmission ratio. The influence of material particles on the operation of the unit for altering the force transmission ratio is therefore reduced in a sustained manner. For said reason, the wear dependence of the actuating performance of the vehicle brakes is also reduced.
The depression for receiving material particles is best arranged with respect to the inner piston and the outer piston in such a manner that it is covered in all functional positions of the unit for altering the force transmission ratio. The depression may e.g. be formed in such an area of the outer piston that it is constantly, i.e. in all functional positions, covered by the inner piston. The depression is in this event preferably formed in a front area of the outer piston facing the reaction disk. The same applies when the inner piston is provided with a depression. In this case, however, the depression should be provided in a rear area of the inner piston which faces away from the reaction disk. In this way it is possible to achieve that material particles that got into the depression stay there. This counteracts unintentional migration of the entire material particles.
The depression preferably takes the form of a groove, for example a groove extending fully or at least in areas in the peripheral direction of one of the pistons. Such a construction of the depression guarantees to a particular extent that migrating material particles may be reliably collected inside the depression. The dimensions of the depression are advantageously selected in such a way that the maximum amount of material arising over the lifetime of the brake booster can be accommodated.
The inner piston may be connected at its end facing the force input element to an e.g. cylindrically constructed plunger. It is then possible for the outer piston to be guided so as to be slidably displaceable on a shank of the plunger. Such a construction removes the need for separate guidance of the outer piston and leads to a compact construction. The plunger and the inner piston are preferably of an integral construction.
To further simplify the structural design and to reduce the size of the unit for altering the force transmission ratio, in preferred embodiments of the brake booster according to the invention the plunger shank is provided at its end facing the force input element with a dish, against which an elastic element may be supported, which preloads the inner piston towards a stop formed on the outer piston. The stop limiting the axial displaceability of the inner piston counter to actuating direction is preferably disposed radially at the inside of the hollow-cylindrical outer piston.
The elastic element preloading the outer piston into its initial position may have a linear spring characteristic. Equally, however, the elastic element may have a progressively rising spring characteristic, with the result that the braking force assistance of the brake booster according to the invention, starting with the movement of the outer piston out of its initial position up to reaching an end position of the outer piston, progressively increases.
The outside diameter of the outer piston is advantageously greater than the inside diameter of the dish so that the outer piston may easily be supported against the stop for the outer piston, the stop being formed e.g. on the control valve housing. Although the dish may also be constructed integrally with the plunger shank, the provision of dish and shank as separate structural components is preferred. Providing dish and shank as separate components allows particularly easy assembly of the unit for altering the force transmission ratio. This is described in detail later with reference to the drawings.
According to a preferred embodiment of the invention, the unit for altering the force transmission ratio takes the form of a cartridge which, after having been preassembed, can be inserted into the control valve housing. The cartridge may be integrated without a high outlay into many pre-existing brake boosters. There is moreover the possibility of offering one and the same brake booster with or without variable force transmission by fitting or not fitting the cartridge into the brake booster.